(1) Field of the Invention
This invention relates in general to electronic imaging and in particular to a technique for correcting for defects in solid state imaging apparatus.
(2) Description Relative to the Prior Art
Solid state imaging, such as by means of a charge coupled device (CCD), has found wide acceptance in the video arts for a variety of reasons, among which are low power consumption, high sensitivity, and potential cost saving as the technology for producing such devices matures. A typical CCD imager, as representative of the general class of imagers in question, comprises an array of a large number of evenly distributed elemental photoresponsive picture elements, i.e. pixels: the greater the number of pixels per imager, the greater the image resolution of the imager. While there is a trend toward employing more and more pixels per imager for image resolution purposes, it is recognized that the greater the number of pixels that an imager has, the greater will be the chance that one or more of the pixels will be bad, thereby possibly rendering the imager useless. Two kinds of pixel defects are generally known to exist in a solid state imager: a "dead" pixel in which little or no image representative photocharge can exist; and a "hot" pixel in which too much photocharge gets accumulated due, for example, to electrical dark current leakage at the pixel. Thus, despite the trend toward more and more pixels per imager, practical design considerations often militate against using more pixels per imager than are absolutely necessary.
Various relatively straightforward techniques are known for obviating the effect of one or more defective pixels in a CCD imager. See, for example, U.S. Pat. No. 4,253,120, issued Feb. 24, 1981, which teaches that, if lower resolving optics for imaging onto a high resolution imager are employed, any given image spot will overlap more than one pixel; and, attendantly, when a pixel is bad, interpolation can be employed to obscure, in real time, the effect of such bad pixel. It is also known to memorize, by use, for example, of a read-only-memory (ROM), the locations of defective pixels of an imager, thereby to produce appropriate correction signals (typically signals corresponding to neutral gray) as substitutes for signals produced by dead and hot pixels. Regardless of which of the aforementioned schemes is employed, correction for defective pixels is usually less than perfect. The latter scheme merely works to dull the effect of a defective pixel; and the former works in a way that, in essence, guesses at what occurs between pixels.
U.S. Pat. No. 4,237,488 teaches a variant of the aforenoted practice of memorizing the locations of defective pixels of an imager, utilizing as "correction signals" signals which correspond to pixels in proximity to defective pixels. In the event, for example, that the 39th pixel of row #10 of an imager is bad, such pixel location is memorized; then, each time the pixel in question is to be read during operation of the imager, the signal from the 39th pixel of row #9 is (typically) substituted for the signal from the bad pixel of row #10. While this technique would appear to be better than the uncorrelated "substitution of neutral gray", it too leaves something to be desired. For example, not only does the teaching of
U.S. Pat. No. 4,237,488 require a precision dynamic buffer for storing prior video information for signal-substitution purposes, but such a tack involves the substitution of information which is only partially correlated with bad pixel information, i.e. the substituted information is not the same as, but only similar to, the defective pixel information. Thus, depending on the number of bad pixels of an imager, information substitution as taught in U.S. Pat. No. 4,237,488 will correspondingly work to blur the image produced by means of the imager. Perhaps more important than either of the aforenoted shortcomings of the apparatus of U.S. Pat. No. 4,237,488, however, the technique of U.S. Pat. No. 4,237,488, when applied to color imaging, requires customization according to the color filter array which is employed to break a scene into its constituent colors. In the event, for example, that the color filter array in question is a column-wise striped filter, it (of course) is okay for there to be the substitution of information from a pixel of a former line for defective information from a similarly numbered pixel of a subsequent line . . . this being because the two pixels in question, by design, correspond with information of the same color; if, however, a color filter array having any of a variety of checkered patterns is employed, it makes little sense to substitute information from a pixel of one line for information from a defective similarly numbered pixel of a subsequent line because such substitute information will, of necessity, correlate with a different color than that associated with the defective pixel. This means, then, that the information substitution scheme of U.S. Pat. No. 4,237,488 must be customized according to the form of color filter array which is employed, a matter which can lead to hardware complexity depending on the information-substitution technique which is required.